Cooling towers are heat exchangers of a type widely used to emanate low grade heat to the atmosphere and are typically utilized in electricity generation, air conditioning installations and the like. In a natural draft cooling tower for the aforementioned applications, airflow is induced via hollow chimney-like tower by the density difference between cool air entering the bottom of the tower and warm air leaving the top. This difference is due to heat transfer from the fluid being cooled, which is passed through the interior of the tower. Cooling towers may be wet or dry. Dry cooling towers can be either “Direct Dry,” in which steam is directly condensed by air passing over a heat exchange medium containing the steam or an “Indirect Dry” type natural draft cooling towers, in which the steam first passes through a surface condenser cooled by a fluid and this warmed fluid is sent to a cooling tower heat exchanger where the fluid remains isolated from the air, similar to an automobile radiator. Dry cooling has the advantage of no evaporative water losses. Both types of dry cooling towers dissipate heat by conduction and convection and both types are presently in use. Wet cooling towers provide for direct air contact to a fluid being cooled. Wet cooling towers benefit from the latent heat of vaporization which provides for very efficient heat transfer but at the expense of evaporating a small percentage of the circulating fluid.
In addition to types of cooling tower designs described above, cooling towers can be further classified as either cross-flow or counter-flow. Typically in a cross-flow cooling tower, the air moves horizontally through the fill or packing as the liquid to be cooled moves downward. Conversely, in a counter-flow cooling tower air travels upward through the fill or packing, opposite to the downward motion of the liquid to be cooled.
In a direct dry cooling tower, the turbine steam exhaust is condensed directly in an air-cooled condenser. Approximately five to ten times the air required for mechanical draft evaporative towers is necessary for dry cooling towers. This type of cooling is usually used when little or no water supply is available. This type of system consumes very little water and emits no water vapor plume.
To accomplish the cooling required the condenser requires a large surface area to dissipate the thermal energy in the gas or steam and presents several problems to the design engineer. It is difficult to efficiently and effectively direct the steam to all the inner surface areas of the condenser because of nonuniformity in the delivery of the steam due to system ducting pressure losses and velocity distribution. Therefore, uniform steam distribution is desirable in air cooled condensers and is critical for optimum performance. Therefore it would be desirous to have a condenser with a strategic layout of ducting and condenser surfaces that would ensure an even distribution of steam throughout the condenser, while permitting a maximum of cooling airflow throughout and across the condenser surfaces.
Another problem with the current air cooled condensers is the expansion and contraction of the ducts and cooling surfaces caused by the temperature differentials. Pipe expansion joints may be employed at critical areas to compensate for the thermal movement. A typical type of expansion joint for pipe systems is a bellow which can be manufactured from metal (most commonly stainless steel). A bellow is made up of a series of one or more convolutions, with the shape of the convolution designed to withstand the internal pressures of the pipe, but flexible enough to accept the axial, lateral, and/or angular deflections. In all but the smallest of applications, branching of the steam ducting is required to distribute the steam to the various coil sections of the condenser. The very nature of branching breaks the steam flow into different directions which necessarily introduces thermal expansion in different directions. These expansion accommodating devices are expensive. Therefore it would be additionally desirous to have a condenser arrangement in which the thermal expansion and contraction is simply and inexpensively managed.
The natural draft cooling tower typically has a hollow, open-topped shell of reinforced concrete with an upright axis of symmetry and circular cross-section. The thin walled shell structure usually comprises a necked, hyperbolic shape when seen in meridian cross-section or the shell may have a cylindrical or conical shape. Openings at the base of the tower structure enable ingress of ambient air to facilitate heat exchange from the fluid to the air. Forced draft cooling towers are also known, in which the airflow is produced by fans. These devices usually do not incorporate a natural draft shell because the fans replace the chimney effect of the natural draft cooling towers. However, forced draft fans may be incorporated in a natural draft design to supplement airflow where the density difference described above is not sufficient to produce the desired airflow.
It is known that improving cooling tower performance (i.e. the ability to extract an increased quantity of waste heat in a given surface) can lead to improved overall efficiency of a steam plant's conversion of heat to electric power and/or to increases in power output in particular conditions. Cost-effective methods of improvement are desired. The present invention addresses this desire. Equivalent considerations can apply in other industries where large natural draft cooling towers are used.
Additionally, large natural draft cooling towers are high-capital-cost, long-life fixed installations, and it is desirable that improvements be obtainable without major modifications, particularly to the main tower structure. The method and apparatus of the present invention are applicable to the improvement of existing natural draft cooling towers, as well as to new cooling towers.
In cooler weather the return temperature of a fluid from the cooling tower and/or freezing a fluid in the heat exchanger is a major concern. When the airflow has the capacity to exchange more heat than desired the airflow must be reduced. Airflow dampers are known to be used is series with heat exchangers. The dampers may be throttled to restrict the airflow. However, even in the wide open position a pressure loss through the damper occurs. This pressure loss reduces the total airflow and thus the cooling capacity of the tower.
Additionally, due to temperature and humidity extremes, a natural draft cooling tower may extract too much heat energy out of the heated liquid or have the liquid to be cooled freeze up. For example, a dry cooling tower may extract too much thermal energy away from the heated liquid condensate, which would require extra heating energy from a boiler or heat source to reheat the liquid back to its optimal temperature, thus lowering the system's efficiency. A wet tower on the other hand is susceptible to ice formation in cold weather. In particular ice may form and build up in the fill and cause structural damage to the fill and/or the supporting structure.
Therefore it would desirous to have an economical, efficient natural draft cooling tower in which the cooling airflow could also be controlled, while keeping the effects of thermal expansion and contraction of the condenser and ducting at a minimum, thus simplifying and reducing the cost maintenance.